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International audience; This article summarizes the major part of the research project SOKA ”Simulation et Optimisation de canoës-KAyaks”, in collaboration with INRIA Sophia-Antipolis and the French Canoe Federation, founded by INSEP (the French National Institute of Sport and Physical Education) and the French ministry ”Ministère de la Ville de la Jeunesse et des Sports”.The main goal was to develop a procedure which includes experimenal measurements and advanced numerical simulation of the flow around the hull to deduce an analytical model of the global loads of the kayaker on his hull. This model was thus used to ad- dress and test an optimisation procedure based on unsteady CFD simulations with solved motions and analyse the influence of some parameters on the forward velocity of the hull.

International audience

International audience; The flow around the well-known KRISO Very Large Crude Carrier no. 2 (KVLCC2) is characterised by boundary layer thickening, flow separation, and vortex creation on the aft body of the ship, leading to a very nonhomogeneous velocity field in the propeller plane (figure 1). It is known since the 1990s that the numerical simulation of such flows is highly sensitive to the treatment of the turbulence. For RANS equations, standard turbulence models perform poorly; measures like the anisotropic treatment of turbulence production are necessary in order to compute correctly the most famous aspect of the flow, the hook-shaped low-velocity zone in the main longitudinal vortex. The case is therefore justly famous as a test of turbulence models for marine flow simulation. The focus of most studies that use the KVLCC2, such as Larsson et al. [3], is the comparison of different flow solvers or different turbulence models. To keep such studies compact and accessible, comparisons are mostly done for a few physical features only. The propeller plane with its hook shape in the axial velocity is often studied. The objective of this paper, on the contrary, is to use a single numerical simulation of good quality to visualise the entire flow field around the aft body of the KVLCC2.We then attempt a physical analysis of this flow, searching explanations of less-studied features such as the outer boundary layer shape and the flow behind the propeller hub cap.We hope that better knowledge of the flow will allow, in the future, to understand more precisely the behaviour of different turbulence models, which may ultimately lead to better models. The paper starts with a definition of the test case and the computation (section 2). Then section 3 presents the visualisation and analysis of the numerical solution. To assess the validity of the analysis, selected aspects of the solution are compared with experiments in section 4. The conclusion (section 5) addresses the challenges for turbulence modelling which the KVLCC2 test case still presents.

International audience; The accurate modeling of surface tension is an important prerequisite to correctly simulate the phenomena that take place in smaller scales. However, modeling surface tension for complex hydrodynamic flows, such as marine applications, poses several difficulties. More specifically the method has to deliver accurate curvature results for absurdly changing topological features of the free surface. As a result, the topological features of the free surface have to be dynamically recovered during the calculation. In this work, we present a surface tension (ST) method implemented in the VOF-RANS flow solver ISIS-CFD [1], that has the above features and present preliminary results obtained for a flat inclined plane cutting the free-surface in an asymmetric flow. In order to capture the geometry of free-surface, the method uses a marching cube algorithm (see for example [2]). The algorithm dynamically generates the grid of the volume fraction isosurface representing the free surface. The curvature is calculated by the generated grid, using a high-order differencing method based on least squares. The results obtained for the test case studied, demonstrate that the method can be used in practical cases to directly visualize complex three dimensional flow regimes and investigate the surface tension effects for smaller scales. For example, the figure below shows the unstructured surface grid generated for the ventilating air vortex emanating near the plate's (transparent plane) submerged leading edge corner and a detail of the free surface roll-up.REFERENCES[1] P. Queutey and M. Visonneau, “An interface capturing method for free-surface hydrodynamicflows”, Computers and Fluids, Vol. 36, pp. 1481−1510, (2007).[2] T.S. Newman, H. Yi, “A survey of the marching cubes algorithm”, Computers and Graphics,Vol. 30, pp. 854–879, (2006).

International audience; This paper studies if adaptive grid refinement combined with finite-volume simulation of the incompressible RANS equations can be used to obtain grid-independent solutions of realistic flow problems. It is shown that grid adaptation based on metric tensors can generate series of meshes for grid convergence studies in a straightforward way. For a two-dimensional airfoil and the flow around a tanker ship, the grid convergence of the observed forces is sufficiently smooth for numerical uncertainty estimation. Grid refinement captures the details of the local flow in the wake, which is shown to be grid converged on reasonably-sized meshes. Thus, grid convergence studies using automatic refinement are suitable for high-Reynolds incompressible flows.

International audience

International audience

International audience

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Submitted to Ocean Engineering on 15 July 2014

International audience; L'objectif de cet article est de d'écrire l'état des travaux aussi bien expérimentaux que numériques réalisés au LMF pour étudier l'écoulement autour des palettes d'aviron. La démarche adoptée, l'influence des paramètres physiques prépondérants ainsi que les étapes vers la modélisation d'un coup d'aviron réel avec la précision adéquate sont ainsi présentés.

International audience; This article deals with the validation of the modelling and numerical simulation of a rowing stroke, by means of CFD. Simplified but realistic strokes were performed in a towing tank with a rotating arm and a real flexible oar. Those laboratory conditions are better controlled than those of in situ trials. An FSI procedure is developed to take into account the oar bending, which is essential in the physics of this flow. The results show that this numerical framework is able to reproduce qualitatively the real flow including the breaking of the free surface around the blade and the transport of the air cavity behind it. The profiles of forces are well reproduced, with propulsive forces overestimated by 5-12% for their maxima. The study also focuses on the computation of the uncertainties. It is highlighted that, even for this well-controlled experimental equipment, the uncertainties on the quantities of interest are of about 11%. In other words, the experimental uncertainty covers the numerical errors. So, this numerical modelling is validated and can be used for design and optimisation of blades and oars, or to contribute to the better understanding of the boat-oar-rower system and its dynamics.

One main concern in climate science is to reduce uncertainties on sea level predictions. In particular,these uncertainties depend on the quantification of the mass losses of polar ice caps including Greenlandice sheet. Iceberg calving at Greenland glaciers accounts for up to half of ice losses at glacier termini.Some icebergs detaching from Greenland glaciers have the height of the glacier and are thin so unstableand capsize exerting a force on the glacier front which generates a seismic wave recorded at stationsin Greenland and further away. These seismic signals have been recorded for the last twenty-five yearsby the permanent stations. The number of such events has increased and the spatial distribution hasevolved. What is the evolution of the volume of capsizing icebergs ?Field data on capsizing icebergs are lacking, except for seismic data. Indeed, the database of seismicsignals gives continuous information about iceberg capsize events: we aim to extract information fromthese data. The characteristics of seismic signals depend on the iceberg volume and the whole dynamicof the capsizing iceberg [2]. The global aim of this work is to calculate the volum of capsizing icebergs.To do this, we compare recorded seismic signals to synthetic seismic signals calculated using a modelof iceberg capsize. Therefore, we solve an inverse problem to obtain information on the dynamic of thecapsize and in particular an estimation of the volume of the iceberg.Iceberg capsize dynamics depends on complex phenomena: iceberg-water interactions, iceberg-glacierfriction, glacier-sea floor friction, elasto-viscoplastic deformation of ice. Solving directly fluid flow, solidmotion, and contact equations even in two dimensions is very costly and can hardly be used to generatecatalogs and to solve inverse problem. Therefore, a simplified mechanical model of a capsizing icebergin water has been developed based on few assumptions. The proposed model, named SAFIM (semi-analytical floating iceberg model) accounts for sea hydrodynamics only through hydrostatic pressure,pressure drag, and added mass, and it has been validated based on a separate state-of-the-art Compu-tational Fluid Dynamics code which can handle free surface and arbitrary iceberg configurations. Theerror on the horizontal force exerted by the fluid on the iceberg capsizing in open ocean (with no contactwith the glacier) calculated with SAFIM goes from 4% to 20%, and the drag coefficient that minimizesthis error goes from 1 to 3, depending on the aspect ratio of the iceberg., Une des questions principales en sciences du climat est l'amélioration de la précision des prédictions du niveau des océans. En particulier, les incertitudes dépendent de la quantification de la perte de masse des calottes polaires comme celle du Groenland. Le détachement d'icebergs au front des glaciers du Groenland représente jusqu'à la moitié des pertes de masses aux terminus de ces glaciers. Certains icebergs qui se détachent des glaciers du Groenland ont la même hauteur que le terminus du glacier, sont fins donc instables et se retournent proche du glacier. Ils exercent ainsi une force sur le glacier qui crée un signal sismique enregistré sur des stations situées au Groenland ou plus loin. Le nombre d'évènements de ce type a augmenté et la distribution spatiale de ces évènements a évoluée [1]. Qu'en est-il de l'évolution du volume de ces icebergs qui se retournent ? Les données de terrain sur ces évènements sont rares, à l'exception des données sismiques qui donnent des informations continues sur les évènements de retournement d'icebergs. Les caractéristiques du signal sismique dépendent du volume de l'iceberg et de toute la dynamique du retournement [2]. Le but ultime de ce travail est de calculer les volumes des icebergs qui se retournent. Pour ce faire, nous comparons des signaux sismiques enregistrés à des signaux sismiques synthétiques obtenus à l'aide d'un modèle mécanique de retournement d'iceberg [3]. Ainsi, nous résolvons un problème inverse pour obtenir des informations sur la dynamique du retournement et une estimation du volume des icebergs [4]. Nous nous intéressons, dans cet article, à la modélisation mécanique du retournement d'un iceberg. La dynamique du retournement d’un iceberg dépend de phénomènes complexes : les interactions entre l’iceberg, l’eau et le glacier, le frottement entre le glacier et le lit rocheux, la déformation élasto-viscoplastique de la glace. Résoudre les équations de la mécanique des fluides, du mouvement solide et les équations de contacts solides est très coûteux, même en deux dimensions, et peu adapté à la génération d’un catalogue pour résoudre un problème inverse. Ainsi, un modèle mécanique simplifié de retournement d’iceberg dans l’eau a été développé sous quelques hypothèses. Le modèle proposé, nommé SAFIM (modèle semi-analytique d’un iceberg flottant) tient compte de la dynamique des fluides via la pressionhydrostatique, une pression de traînée et des masses ajoutées. Ce modèle SAFIM a été validé à partird’un modèle de mécanique des fluides numérique ISIS-CFD qui résoud la dynamique d’un fluide à surface libre en interaction avec un solide. L’erreur sur la force horizontale du fluide sur l’iceberg qui se retourne dans un océan infini (sans contact avec le glacier) calculée avec SAFIM est de l’ordre de 4% à 20%, et le coefficient de traînée qui minimise cette erreur varie entre 1 et 3, en fonction du rapport d’aspect de l’iceberg.

International audience; This paper studies if adaptive grid refinement combined with finite-volume simulation of the incompressible RANS equations can be used to obtain grid-independent solutions of realistic flow problems. It is shown that grid adaptation based on metric tensors can generate series of meshes for grid convergence studies in a straightforward way. For a two-dimensional airfoil and the flow around a tanker ship, the grid convergence of the observed forces is sufficiently smooth for numerical uncertainty estimation. Grid refinement captures the details of the local flow in the wake, which is shown to be grid converged on reasonably-sized meshes. Thus, grid convergence studies using automatic refinement are suitable for high-Reynolds incompressible flows.